1. Field of the Invention
This invention relates to a method and apparatus for heat-treating nonwoven webs.
2. Description of Related Art
Methods for drying and heat-treating sheet-type materials such as nonwoven, knitted, and woven fabrics are known in the art. For example, air-impingement and flotation dryers are known in the art that are suitable for drying sheet materials that can tolerate the relatively high tensions used to pull the sheet through the process. Porous sheet materials can also be heated while under low or substantially zero tension by pulling a heated gas, such as air, through the fabric while pinning the fabric to a porous surface such as a drum or belt. For example, in a through-air bonder hot air on one side of a sheet is pulled through the sheet by applying a vacuum to the opposite side, the vacuum also serving to pin the sheet to a porous surface on which the sheet is supported. It is also known to remove residual water from a nonwoven fabric that has been topically treated with a chemical finish composition by passing the fabric over steam cans.
An air levitation dryer developed by Mascoe for processing coated webs is described in Journal of Coated Fabrics, Vol. 25, January 1996, pp. 190-204. The levitation system is described as having the ability to support webs up to 60 inches wide and 50 feet in length without using a transport conveyor or tenter and with no contact to any supporting surface using no more than 10 pounds lineal tension. Such dryers have the limitation that they are not readily adapted to high processing speeds. Other low-tension dryers are disclosed in International Dyer, 185, Number 3, p.27 (March, 2000). In one example, a fabric is transported in a tensionless state using a conveyor belt and overfeeding the fabric, alternating between sections where the fabric is run in a wave by means of alternate upper and lower air flows and sections where suction is performed under the belt.
However, known drying processes can cause defects to form in the nonwoven webs during the drying process, such as waves or puckers in the fabric sheet, especially when the polymeric fiber component(s) in the fabric are relatively low-melting temperature materials.
It would be advantageous to be able to dry and/or cure chemical finishing agents applied to a nonwoven web or sheet which comprises relatively low-melting temperature polymeric fiber components, or otherwise heat-treat such a nonwoven web or sheet, without creation of defects in the dried nonwoven sheet or web.